Mechanisms of placental transfer of mineral elements will be investigated using the pregnant guinea pig as the model animal. Experiments will be conducted using intact animals and surgical preparations in which the fetal circulation of the placenta is perfused. Cardiac output and blood flow distribution measurements will be performed as a function of fetal age and litter size. The electropotential gradient from dam to fetus will be determined as will the maternal blood and perfusate pCO2 and pH. Deconvolution studies will be performed to determine the bidirectional fluxes of Ca across the placenta.